Refractory body containing boron nitride



United States atent ice REFRACTORY BODY CONTAINING BORON NITRIDE KennethM. Taylor, Lewiston, N. Y., assignor to The Carbonmdum Company, NiagaraFalls, N. Y., a corporation of Delaware No Drawing. Application ltiarch12, 1956 Serial No. 570,667

Claims. (Cl. 106-44) This invention relates to bonded boron nitridebodies and to compositions and methods of making said bodies.

This application is a continuation-in-part application of my applicationSerial No. 288,551 filed May' 17,1952, now abandoned.

There is a constant search for new compositions or k bodies which willpossess unexpected combinations of properties essential to or generallyfound to be desirable in specific fields of use. The bonded boronnitride bodies ing characteristics as a refractory material areparticularly worthy of note and make them especially suitable for anumber of refractory purposes. The present invention will therefore beprimarily described in respect to the use of the herein describedproducts for refractory purposes, although not intended to be limitedthereto.

Above all, a refractory body must possess refractoriness, that is, anability to stand up under exposure to high temperatures without unduechemical or physical change. Other desirable characteristics sought in arefractory body or shape include an ability to resist sudden changes intemperature without cracking or other manifestations of body breakdown,a relatively high mechanical strength at elevated temperatures as Wellas at room temperature, chemical inertness and resistance to variouscorrosive and erosive substances and conditions, a resistance tooxidative influences, and a density and hardness dependent upon the useto which the refractory body is to be put.

In order to obtain a high degree of perfection in respect of one or moreof the above properties peculiarly desirable for the specific purpose inmind it has usually been found necessary to forego the benefits ofrnaximum performance in respect of certain other desirable properties.Consequently, various refractory compositions exceptionally suitable forone field of use are often found. .to be entirely unsatisfactory forother purposes. There is, therefore, a continual demand for refractorybodies of new composition which will meet those demands of a specialnature which require a combination of properties not to be found inthose compositions of a refractory type already available. There is alsoa demand for refractory compositions which can be made readily withoutresort to exceedingly high temperatures.

It is an object of the present invention toprovide bonded boron nitridebodies or shapes of unusual and distinctive compositions and properties.

It is another object of the present invention toprovide refractorybodies or shapes having a particular combination of refractoryproperties heretofore unavailable in refractory compositions.

It is another object to provide compositions of matter and methods formaking new and distinctive, bonded boron nitride bodies or shapes.

. Other objects and advantages accruing from the present invention willbecome apparent as the description proceeds.

In accordance with nitride shapes or bodies are formed by mixing boronnitride in granular or powdered form with finely divided silicon metalor finely divided aluminum metal, with or without the addition of asmall amount of a temporary binder orplasticizer to provide green moldedstrength, compressing a mass of the material or forming an article ofthe desired shape by any of the well-known methods of formation such aspressure molding, tamping, slip casting, extrusion or the like, dryingthe formedarticle and firing it in an atmosphere of ammonia, nitrogen orother non-oxidizing atmosphere containing nitrogen at a temperature andfor a period of time suflicient to convert the silicon or aluminum tosilicon nitride or aluminum nitride. Granular or powdered siliconcarbide may or may not be included inthe mixture from which the articleis made.

In order to efficiently convert the silicon or aluminum metal to siliconnitride or aluminum nitride the metal should be in the neighborhood of200 mesh (U. S. Standard Sieve) size or finer. The silicon nitride oraluminum nitride which is formed in situ from the silicon or aluminummetal serves as a interstitial bond to strongly unite the boron nitridematerial to provide a body of satisfactory mechanical strength, theamount of silicon nitride or aluminium nitride in the final articlebeing dependent upon the amount of silicon or aluminum metal used in theraw batch. The particular proportion of boron nitride to silicon oraluminum will depend upon the particular type of article desired and thepurpose for which the resulting product is intended. When bodies are tobe made consisting predominantly ofaluminum nitride with a lesser amountof boron nitride, it is preferred to form the body by a slightmodification of the same general procedure in which the boron nitridewould -b'e initially mixed and molded with part of the aluminum andnitrided, the resulting body crushed and mixed with the balance of thealuminum and molded andnitrided to provide a body having the desiredamounts of boron nitride and aluminum nitride. j Y 7 I have found thatwhere. silicon is used to provide a silicon nitride bond for the boronnitride, highly satisfactory results are to be obtained by using acommercial grade silicon ground to suitable fineness. Analysis of onecommercial grade of silicon which I have satisfactorily used in carryingout the present invention discloses,-in addition to the silicon, thepresence of the following impurities:

Percent Ir-on 0.87 Chromium 0.21 Aluminum 0.60 Calcium 0.54

The boron nitride used in making the bonded boron nitride bodies orshapes of the present invention may be a commercial gradepf boronnitride material available on the market. However, I prefer to use aboron nitride material made in accordance with the process described inmy copending application Serial No. 288,553, filed May 17, 1952, issuedOctober 1, 1957 as U. S. Patent No. 2,808,314. Briefly, that process maybe described j as comprising the preparation of a porous, pelletedmixture of boric acid or boric oxide and tricalciurn phosphate, thepelleted mixture being heated in an atmosphere of ammonia in an electricmuflle furnace or other suitable muffle furnace for several hours ataround 900 C.

whereby the boric oxide is converted to boron nitride,

the present invention bonded boron The resulting material is crushed andtreated with dilute Hydrochloric acid to dissolve the tricalciumphosphate and other extraneous material. The undissolved boron nitrideafter several washings with water is usually treatedwithhot 95% alcoholsolution to further 'lower theeohteht 'o'f oXidic material. Analysis ofthe resulting boron nitride is as follows:

Percent Boron 41.45 Nitrogen .a 44.00 'Freeboricacid (calculated as H BO.75 Silica .28

Calcium Phosphate (P I Material volatile at 110 C. 'Extf aneousn'i'atter 13.26

The 13.26% of extraneous matter in the above table of analysis (if the'boron nitride product has not been :fully identified as to characterbut insofar as it has been able to be determined it is considered to befor the'most part-oxygen which is eitherphysically absorbed or unitedtothe boronnitride in such a way that it is not alcoholsoluble as wouldbe the case if it were present in the :form of boric oxide. Althoughthe-material'before bein hot pressed into :a shaped body does notcontain any alcohol-soluble boric oxide, the shaped bodies resultingfrom hot'ipressing the material are found to contain a certainamount'oftree boric oxide. It is therefore concluded-that a certainamount of a physically or chemically combined oxygen complex iscontained in the original material, although X-ray analyses reveal thepresence only of boron nitride. The analysis given above is there forecomplete-as-far as 'it has been possible to positively identify thecomposition.

Bonded boron nitride bodies or-shapes'have been made 'in accordance withthe presentinvention in the following manner. The boron nitride ismixedin the desired proportions 'with--finely divided silicon metal oraluminum metal, depending upon whether a silicon nitride bond or 40 'l'fie'thus-for-m'ed articles are thendried and fire'd'a't a temperature-ofaround 1400 C. in a nitrogen-containing, nch-oxidizing,substantiallynon-carbonaceous atmosphere. The temperature can be'raise'cl somewhatabove 1400 C.-dur-ing the latter part of the nitriding operation afterthe nitriding reaction has progressed'for some period of time to furtherassure completion of the nitriding action. Although commercial nitrogengas is usually employed, ammonia gas or annealing hydrogen (containing93% nitrogen-and 7% hydrogen) eanbeused-with satisfactory results. It isessential, however, that the nitrogen-containing gas be substantiallynon-oxidizing in character. It is also desirable where a silicon nitridebond is desired that the nitrogenous gas atmosphere be substantiallyfree from materials which provide elements other than nitrogen whicharereactive 'With the silicon or aluminum. Such materials includecarbonaceous materials, such as carbon dioxide, carbon monoxide,hydrocarbons such as methane, or other constituents providing a sourceof elements such as oxygenor carbon which have a tendency to unitereadily with silicon or aluminum. However, the presence ofsli'ghtamounts of carbonaceous substances as impurities in thenitrogen-containing atmosphere can be tolerated and is not to beconsidered as a departure from the present invention.

In order that the invention may be clearly understood, the followingexamples are submitted as-illustrative of the compositions for andmanner of carrying out the present invention:

EXAMPLE I Small-nozzles 1% l11Ch6S inlength and inchin outside diameter,bar-shaped compacts and other molded shapes and articles composed ofboron-nitride held -together by abo'n'd of silicon nitride have beenmade as follows. An intimate mixture was made of 5.1 parts by weight(85%) of'finely divided boron nitride and .9 part by weight (15%)'of'finely divided 'silic'on "metal having a particle size of 200 meshand'finer. To the mixture of boron nitride and silicon metal *wasadded'5% by weight of the total-mass er carbowaif No. '4000 as atemporary binder "and "the resulting mixture molded into the desiredshape "at apres'sure of from 5,000 to 30,000 pounds' per squa'reinclrdependin'gupon the particular shaped article being "made.*Ac'c'ordirig 'to the Hand Book of Material Trade Names "by Zimmermanand Lavine (published by industrial Research Service, Dover, NewHampshire 1953') "page '110, Caibowaf'i's a group of non-volatile,solid'polyethylen'e Tglyc'ols, "soluble in both water andaromatichydrocarbons. They resemble natural Waxes in appearance'andtexturejhutare soluble in a much wider range 'ofsolvents. Theiraqueous solutions possess binding'prop'erties. The "same source ofauthority states that Carbowax No. 4000 is a'hard, waxy solid havingspecific gravity "of 1.2, "freezing ra'nge ofSO-SS" C., a'flashpoin'tgreater than 475 F, anda Sayb'olt viscosity of 500-700 seconds "at .210'F. The temporary binder was "removed by heating the molded shape for afew hours at 300- 400" C. The resulting shape was then fired in anatmosphere of nitrogen or amm'oniaat 1400" C. for a sufficient perio'doftime' to convert the silicon in the body to silicon nitride.

Table I belowjpresents the fabricating data and also some of thephysical properties-of various bar shaped compacts composed of mixturesof boron nitride and silicon fired in ammonia or nitrogen at 1400 C.

Table l BAR S HAPED GOMPAOTS OF MIXTURES OFLBDRON NITRIDE AND SILI- CONFIRED IN AMMONIA OR NITROGEN AT 1,400" O.

, 4 v Pressure Sandblast Apparent Experiment Raw Mix Com used in'peuetra density No. posltlon -perc'ent forming, Firing Conditions Hon 1of of fired by weight p. 5.1. fired body body, inches gJcc.

. 011 85 EN; 15 $1.... 30, 000 00s. .0 3 1. 72 BN; 20 Si.-- 30. 000 086'1.03 SO-BN; 20 l.- 30, 000 8 hrs. in N2 010 1:70 BN; 15 Si 20, 000 14hrs. in NH3 .015 1. 44 60 EN; 40 Si 5, 000 6-8 hrs. in N2 047- 1.03 50EN; 508 6,000 fihrs. inN2-- 018 1.91 35 EN; 65 Si--. 5, 000 6-8 hrs. inN .028 1. 84 10 EN; 81.-.; 5,000 6'8'hr'S. in N1"... .012 1:06

1 Standard penetration on plate glass when Subjected to the samepenetration'tsfls 1010 Of an inch.

The bar-shaped compacts set forth in Table I were 1%" in length x 56"wide it $4" to 16" thick. 5% to 7 /&% Carbowax No. 4000 was used as atemporary binder. The compressive strength taken on test specimens inthe form of 16" cubes cut from the resulting bars showed the bodies tohave compressive strength at room temperature of from 5,500 pounds persquare inch to a compressive strength as high as 15,000 to 16,000 poundsper square inch.

Similar a" cubes were subjected toan oxidation test consisting ofheating the cube in for 24 hours at 1000" C. and the weight changedetermined. Such pieces as a result of the test showed a change inweight of around 2.5%.

The bars were also subjected to a heat shock resistance test whichconsisted of placing a specimen bar 1%" x /2" x /z in size in a furnaceat a temperature of 1850 F. until the temperature of the specimenreached furnace temperature. The test piece was then removed and cooledwith a blast of air to a dark color. The heating and cooling wasrepeated through 25 cycles. Heating of the piece required about threeminutes and cooling of the piece required about one minute. The barsremained sound and unaliected after exposure to 25 cycles of such atest.

One-half inch cubes of the same composition were subjected to a loadtest at 1200 C. and showed no slumping as a result of the test whichconsisted of heating the cube to 1200" C. (2192 F.) at a rate of 200 C.temperature rise per hour under a load of60 pounds per square inch. Theloaded specimens were held at 1200 C. for one hour and then graduallycooled to room temperature. Samples were then measured to determine theamount of slump. No slumping occurred on articles of the type madeaccording to Example I and described in Table I.

The bond in the fired pieces such as those shown in Table I has beenshown to be principally silicon nitride as verified by X-ray difiractionanalyses.

Nozzles 1% inches long by inch in diameter were made from the followingcompositions:

No.1, No. 2,

percent; percent Boron nitride 83 65 Silicon 17 35 No. 1, 1.39 g./cc.;No. 2, 1.45 g./cc.

EXAMPLE II Two bar shapes, 1%" x $6" x approximately .3" were formed bypressing mixtures of boron nitride and powdered aluminum at 30,000pounds per square inch with 5% Carbowax No. 4000 as a temporary binder.

Fabricating data and physical properties of the resulting bodies is setforth in Table II below:

AMMONIA OR NITROGEN Weight Sandblast Bar Raw Mix Comgain in penetra- No.position, percent Firing firing, tion 1 on by weight percent tired bars,

inches 1 BN; 15 AL-.. 1% hrs. in nitrogen at 3. 9 :097

1,000 O. i Same bar refired 10 2. 9 .060

hrs. in ammonia at 1,300 O. 2 70 BN; 30 AL... 16 higgn ammonia at; 12.0018 1 3 60 EN; 40 AL... 6 hrs. in nitrogen at .007

1 Standard penetration on plate glass is .010 of an inch.

The resulting bodies have been subjected to several: months aging inairwithout deterioration. Although they were not as hard as the bodiesmade from comparable mixtures of boron nitride and silicon heated inammonia they were of adequate mechanical strength for numerous uses.

Other proportions of aluminum to boron nitride can be used and the timeand temperature of firing varied depending upon the properties desiredin the product.

'Also the firing can be done in an atmosphere of nitrogen instead ofinammonia.

EXAMPLE HI A rocket nozzle 1%" long and '%i" in diameter composed ofboron nitride and silicon carbide held together by a silicon nitridebond was made from the following mix:

Parts by weight Boron nitride 42.5

Silicon .carbide, 220 mesh 32.5 Silicon carbide, colloidal 10.0 Silicon,200 mesh and finer 15.0

To 24 grams of the above mixture there was added 2.4 gramsof CarbowaxNo. 4000 dissolved in 4.8 cc. of

benzene and the mixture ground in a mortar until the heating to about600 C. and the nozzle fired for 8 hours in ammonia gas at approximately1400 C. In firing, the nozzle gained weight and increased in strengthand hardness due to the conversion of the silicon to silicon nitride.Nitrogen may be used in firing instead of ammonia and the proportions ofingredients may be varied over a considerable range. For example, byincreasing the proportion of silicon and consequently the resultingsilicon nitride in the final fired object, such as increasing the proportion of silicon in the original mix to 30%, an article having greaterstrength and hardness is obtained.

Among the advantages derived from a refractory body of the above typecontaining a large amount of boron' nitride over a similar articlecomposedentirely of silicon carbide bonded with silicon nitrideis thegreater lightspecific percentages by weight of boron nitride, silicon'nitride, silicon carbide and aluminum nitride in the bodies made, thepresent invention is not limited to any particulat proportions of thenamed ingredients since the exact proportions of ingredients are notcritical to the invention, it only being essential that suflicientsilicon nitride and/or aluminum nitride be present to impart therequired mechanical strength and hardness to the final 2' articledepending upon the ultimate use to which the article is to be put. As amatter of fact, the silicon nitride or aluminum nitride can amount toaslow as around 895 '7 by weight or as high as around 97% by weight ofthe body, which would require around silicon and around 95% silicon,rmpectively, in:- the raw batches from which the articles were made.Also, any proportion of the boron nitride of the body can be replaced bygranular silicon carbide as a filler, although the body should containat least 5% by weight of boron nitride.

While I have described in the above examples the making of variousmolded shapes in which the article is molded from a mixture of boronnitride and the metal to be nitrided in the exact shape or form in whichit is intended for use, the present invention is not intended to be sorestricted. Another way of making and using bonded boron nitride bodiesof the present invention is to' mold the raw batch of material intobriquettes or shapes or otherwise compress a mass of material having acomposition of the desired proportions, after which the resultingbriquette or compressed bodies are nitrided in the manner alreadydescribed. After removal from the furnacethey are crushed to-granularform of the required grit size. used-in loose granular form at hightemperatures asan insulation material, as for example, insulation aroundrocket combustion chambers, or as a layer of insulationaroundi'ndustrial furnace chambers. It may also be'used as a loosefiltering media or as a catalyst carrier material. The granular materialcan also be bonded by means of si'ntered'meta-l's, vitreous or'oeramic'bonds, or other'bonding materials to form articles suitable for many ofthe industrial uses set forth elsewhere herein.

Likewise, articles or bodies can be made according to the presentinvention in which pore-forming materials are incorporated in the rawbatch from which the body is made for the purpose of providing a greaterdegree of porosity in the final body. Pore-forming material such ascarbon or the like, which requires oxidation for removal from a' body,wouldrequire a preliminary burning out of the pore-forming material atlower temperaures. Therefore, the pore-forming material preferablyshould be a material which is removed by volatilization during thedrying and/or firing operation such as powdered or granular naphthalene,various organic resinous materials such as, phenolic resins and the likeor one which provides pores, by reason of the generation of a gas; Theresulting bodies, which have greater porosities than. those made with nopore formers, are particularly useful in the fabrication of porousfiltering media, catalyst and catalyst carriers, insulation. bodies andthe like, whether in crushed, granular form or in the form of moldedshapes of predetermined contour.

It. is to be understood that the products ofthe present invention in itsvarious modifications are not limited to any specific field or fields ofuse such as might be defined by the specific examples previously setforth. The product can be made-in any desired shape as well as providedin granular or aggregate form.. They are, therefore, not only suited formany of the usesv for which industrial refractories are required,including bricks, blocks, setter tile, muffles, kiln furniture, andspecial shapes for application in and around furnaces and other hightemperature equipment, but they are also well suited for many specialtyhigh temperature applications, such. as jet engine combustion chambers,linings for exhaust nozzles, rocket combustion chambers and exhaustnozzles, turbine blades, stator blades, radomes for guided missiles,lens fusion blocks, spark plug bodies and the like. The bodies are alsosuitable for making crucibles and other laboratory ware or industrialstructural articles or parts for the handling of corrosive chemicalssuch as molten cryolite or other fused halides. fabrication oflaboratory ware including combustion boats, crucibles, burner holdersand other shapes. The bodies of the present invention particularly whenmodified. by the use of pore formers inthe rawbatch from which thebodies are made are also highly useful as The resulting granularmaterial can then be They are also suitable for the diffusion andfiltering media such as. diifusion tubes and plates, filtering tubes,.pla tes. and. shapes, or as catalyst or catalyst carriers and supports.Materials and articles of the present invention can also. be used inmaking abrasive articles such 'as grinding wheel's, sharpening stones,razor bones and other grinding and polishing shapes and material's; Thepr-esent' bodies offer possible applications in theelectrical-and radioindustry including supports in electric: light bulbs, radio tubes, X-raytubes, radar equipment, resistors and grid leaks.

Having. described the present invention it 'isdesired to claim:

I claim: I V

A bonded boron nitride body consisting essentially of 3% to 92% byweight boron nitride and"97'% to 8% of a silcon nitride bond.

2. Asa new article of manufacture; a body of 3% to 92 by weight- -boronnitride anda; bond consisting essentially of 97% to 8% silicon-nitride:

3.. As. a. newarticle of manufacture, a bodyof 3 %v to 92% by weightboron nitride and a bond consisting.essentially of 97% to 8% aluminumnitride.

4. A refracton body consisting essentially of 3% to 92% by weight boronnitride and 97% to 8% by weight silicon nitride.

5. A refractory body' consisting essentiallywf' 3% to- 92%- byweight-boron nit1idean'd -97%- to 8%- by weight aluminum nitride.

6. A. refractory body consisting essentially of. 3%- to 92% by.weightboroir nitride and 97% to 8% by weight of a nitride selected fromthe group consisting of silicon nitride andaluminum nitride.

7. A refractory body according to claim 6' alsocontainingsiliconcarbide.

8-; As a new article of manufacture, -a body consisting essentially of.boron nitride; particles held together-by 8%: to. 97% by weight ofsilicon nitride.

9. Asa new article of manufacture, a body consisting essentially ofboron nitride particles and silicon carbide particles held together by8%' to 97% by weight of silicon nitride;

10. A raw batch for the manufacture of bonded boron nitride bodies, saidbatch consisting essentially of 5% to by weight. of boron. nitride, andfinely divided silicon.

11. A raw batch for the manufacture of bonded boron nitride bodies, saidbatch consisting essentially of 5% to 95% by weight of boron nitride,and finely divided aluminum metal.

12. A raw batch for the manufacture of bonded boron nitride bodies, saidbatch consisting essentially ofboron" nitride, silicon carbide and 5% to95% by weight of finely divided silicon.

13. A raw batch for the manufacture of bonded boron nitride bodies, saidbatch consisting essentially of boron nitride, silicon carbide and 5% to95% by weight of finely divided aluminum metal.

14. A raw batch for the manufacture of bonded boron nitride bodies, saidbatch consisting essentially of boron nitride and 5% to 95% by weight offinely dividedchemical element, saidchemical elementbein'g selected fromthe group consisting ofaluminum and silicon;

15. A method of making bonded boron nitride-articles which comprisesforming a mixture consisting essentially of boron nitride particles and5% to 95 by weight of a finely divided chemicalelement selected from thegroup consisting of silicon and aluminum, molding an article from saidmixture. andfiring said article in a non-oxidizing, nitrogenousatmosphere at a temperature around 1400 C. to convert thechemicalelement to a nitride of said chemical element "and therebybondthe boron nitride particles'together.

'l6.{ A methodof making bonded boron nitride articles. which comprisesforming a mixtureconsisting essentially of boron nitride particles and5% to 95 byweight of Q ,1 a finely divided chemical element selectedfrom the group consisting of silicon and aluminum, molding an articlefrom said mixture, and firing said article in a non-oxidizing,nitrogenous atmosphere to a temperature of around 1400 C.

17. A method of making bonded boron nitride articles which comprisesforming a mixture consisting essentially of boron nitride particles and5% to 95% by weight of a finely divided chemical element selected fromthe group consisting of silicon and aluminum, molding an article fromsaid mixture, and firing said article in an atmosphere of ammonia at atemperature around 1400 C. to convert the chemical element to a nitrideof said chemical element and thereby bond the boron nitride particlestogether.

18. A method of making bonded boron nitride articles according to claim15 in which a part of the boron nitride of the mixture is replaced byparticles of silicon carbide.

19. A method of making bonded boron nitride articles of manufacturecomprising forming an article of the de sired shape from a mixtureconsisting essentially of 5% to 95% by weight of finely divided silconand 95% to 5% by weight boron nitride, and firing the formed article ata temperature of 1400 C. in a non-oxidizing atmos' phere containingnitrogen to convert the silicon to silicon nitride.

20. A method of making bonded boron nitride articles of manufacturecomprising forming an article of the desired shape from a mixtureconsisting essentially of 5% to 95 by weight of finely divided aluminummetal and 95% to 5% by weight boron nitride, and firing the formedarticle at a temperature of 1400 C. in a nonoxidizing atmospherecontaining nitrogen to convert the aluminum metal to aluminum nitride.

ReferencesCited in the file of this patent UNITED STATES PATENTS

1. A BONDED BORON NITRIDE BODY CONSISTING ESSENTIALLY OF 3% TO 92% BYWEIGHT BORON NITRIDE AND 97% TO 8% OF A SILCON NITRIDE BOND.